LEGO Compatible Strandbeest
by rswarner, published
This is a leg pair, derived from Theo Jansen's Strandbeest, that is compatible with LEGO Technic. I built a set of these to give to my six year-old, who is pretty excited about robots and our 3D printer. Combined with two LEGO Technic motors, some geartrain parts, and the LEGO IR remote, this becomes a walking robot. I assembled this as a steerable robot with the drive train in the center, and three leg-pairs on each side. The LEGO remote can drive two motors, so I set it up with one motor driving each side, so that the bot can be steered like a tank.
Here's a short video, narrated by my 6 year old: http://youtu.be/0TVsqX-lvMk
To make a walking robot, you will need six leg pairs.
The Strandbeest leg pairs are very fun to watch walk, and though the leg assemblies in each pair are symmetrical, their movements are not! The leg pairs are modular, and they snap together with four snap pins, and an indexed shaft. This allows a modular assembly that carries the crankshaft through each leg pair. The crank washer detail image shows the position of the washer (grey in the image) in between the connecting rods and the crank disk.
The gait of the Jansen legs is designed so that three leg pairs are the equivalent of one wheel. So my design has you coupling the leg pairs at 120 degrees from one another. To make a complete walking bot, you will need six leg pairs (twelve legs). I will put together some parts plates soon to simplify this.
You will need to print (per leg pair):
1 ea Body Back Shell
1 ea Body Cap
1 ea Body End
1 ea Crank
2 ea PelvisHipA
2 ea PelvisHipB
2 ea KneeFootA
2 ea KneeFootB
2 ea PelvisToC
2 ea BackOfLegToC
4 ea KneeToHip
4 ea Claw-Grooved
1 ea Washer
There is also some hardware, and of course some LEGOs required to make this work. Specifically, you will need some Technic (the blocks with holes through them) parts, indexed shafts (48 mm, black or grey with a + shaped profile), and several little black snap pins. A pair of motors, some gears, and the IR remote go really well with this! I will upload a photo of this group of required parts soon.
I have work to do on the part names. These are derivative of the way they are named here: http://geogebrawiki.pbworks.com/w/page/11911516/Strandbeest
The geometry of each leg is comprised of two right triangles (actually two triangle sandwiches), and two identical struts that form a parallelagram between the two triangles. The points of the upper triangle are the pelvis, hip, and Point A. The points of the lower triangle are the Knee, Ankle, and Foot. I'm sure this will be confusing, but refer the the site above for an explanation. Point C is where all the connecting rods attach to the crank with the indexed LEGO shaft.
These files should be printed with 0.35 mm layer heights. The Z axis dimensions are all multiples of this number so that you can get very close to a LEGO-compatible dimension in all axes. I use a Printrbot+ with a 0.5 mm nozzle, and I have printed this in ABS. I used 25% honeycomb infill with two skin layers with good results. For strength, orient the long parts on your print bed so the grain of the skin layers is diagonal compared to the long axis of the part (my printer will do this if the parts are aligned to the Y axis). The + shaped holes for the indexed shafts are a tight interference fit, and the larger round holes should provide a running fit on those shafts. When you go to assemble the crank disk in the body back shell and body cap, you may have to do some cleaning of the parts with simple hand tools. A small amount of synthetic bicycle grease inside this assembly really helps keep this running smoothly. There are so many points of friction in these assemblies that you need to pay close attention to how tightly you assemble the joints. It all adds up. But so does the slop, so don't go too loose either!
This is designed to be assembled with 6-32 hardware. Some of the parts have "A" or "B" in their names. You'll see that the "A" types have holes that give a slip fit on the 6-32 screws, whereas the "B" versions have a sort-of self-tapping fit. In addition to the 6-32 hardware, you will need four (per leg pair - two per foot) #11 neoprene O-rings, available in the plumbing aisle of your home improvement big box. These provide traction for walking on slick surfaces. The "claw" part is meant to be secured tightly in the foot, not to roll. I just settled on O-rings for simplicity. I will upload an image of the required 6-32 hardware soon. Most of the leg assembly uses 1/2" pan head screws. The Body Cap and Back Shell are held together with 3/8" countersunk screws. The joint where the Back of Leg to C connecting rod connects to the Ankle (third angle on the KneeFoot part) uses a 3/4" pan head, which is a little too long but it works. The Body assembly is held together with two 1 1/4" pan heads, with the heads flush in their counterbores.
6-32 hardware (per leg pair assembly):
8 ea 6-32 1/2" pan head
2 ea 6-32 3/4" pan head
2 ea 6-32 1 1/4" pan head
4 ea 6-32 3/8" countersunk
Orientation of the screws (and the corresponding "A" and "B" triangular parts):
The detail photos of the printed parts show the orientation of the screw heads in the leg assemblies. NOTE: these prints were a previous revision, so some parts look slightly different in these photos, but the assembly process is the same. All of the screws in one leg will go in from the same side. In the assembled leg pair, one leg is flipped. One will have all of its screw heads on one side, and the other will have its screw heads on the other side.
The only part that requires support material is the Body Back Shell, due to the counterbores for the pan head screws. The small counterbores for the LEGO snap pins seem to work fine without support material. YMMV with extruder nozzles smaller than 0.5 mm.
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